Absolute Minimum Requirements

VDR should run starting from 200MHz if a DVB tuner with embedded hardware MPEG-2 decoder is used, a so called Full-Featured DVB card. The processor is only used to give the data from DVB card to hard disc and/or from disc to DVB card. The data transfer rate from and to the hard disc is relatively low (~500Kbyte/s) and slow computer does not have problems with that.

On the other hand, if you don't want to use only MPEG-2 (DVB, DVD), but also MPEG-4 (DivX), the hardware decoder is not able to do that. The CPU has to do this job. In case you bought a DVB tuner without hardware MPEG-2 decoder, a so called Budget DVB card, the main CPU has to decode MPEG bit data stream, even MPEG-2.

600 MHz => too slow for MPEG4/DivX or MPEG-2 software decoding

1 GHz => might be sufficient in some cases

2 GHz => ok

Encoding or transcoding of video needs CPU power. This is valid also when producing VCDs, SVCDs
out of DVB records or audio MP3 encoding. Either the PC used for VDR is fast enough or it should be connected to a faster server computer which does this job.

Choosing the right CPU for you

In general:

slow clock frequency => noiseless, because of no cooling problems

high clock frequency => noisy, because of cooling problems

Passive cooling is possible up to approx. 1GHz (depending on CPU type) or in dissipated power up to 10 Watts, with very big and heavy CPU coolers and case coolers up to 25 Watt is possible. More needs very sophisticated cooling solutions like heatpipes or water cooling.

Helpful in this question might be the dissipated power, which you can find here (German only)